This is the key insight that, once made, pulls utilitarianism out of a lot of its problems. It's not that maximizing utility is bad (when you have a sensible utility measure--often you do even if sometimes it's not clear); it's that you don't have enough information to make a decision now. So you should defer the decision until you get more information. How should you act in the meantime? Why, in ways that generally lead to good outcomes even in the absence of specific information...which...is...suspiciously like virtue ethics.